It may be expansion and contraction of the waste lines. ABS pipe, the black plastic pipe, will move quite a bit as it warms and cools. When the pipes are tight against wood or strapping, you will get those "ticking" sounds. Common places you will find the problem, at where the two studs at the top of the wall are drilled. If the hole is not drilled vertical, the pipe may bind as it "wedges" it's way through. Terry

: We recently purchased a home (3 level split) built in 76 in Mnpls, MN. When you run the water in the upstairs master bath (sink) a few moments later in the bath area opposite the sink (toilet and shower stall in a separate room)a sound like dripping or ticking occurs in the wall behind the toilet. This sound also occurs after the toilet is flushed and refills. Sounds just like water dripping a long distance down. But there is no evidence of any leaks. The sound occurs at all times of the year... isn't related to our cold climate, it seems. The ticking lasts for a couple of minutes maybe 20 ticks or so. The plumbing in the house is copper supply and that ugly black plastic drainage pipe... Any ides as to what may be causing the ticking sound we hear.